There are known systems for measuring an electrical charge using a charge integrator. A typical structure of this type is shown in FIG. 1. The system comprises a capacitance detector 110 connected to a charge integrator 120, which is an operational amplifier with capacitance feedback 130. Measurement of electrical charge q in a system of this type is based on transferring a charge accumulated in a detector system to a reference feedback capacitance Cf and measuring the charge of this capacitance using the equation:
  u  =      q          C      f      
The capacitance Cd of the detector 110 is connected to the input of the charge integrator 120, which has an input impedance. This input impedance has a capacitive character and comprises mainly the dynamic capacitance of the system Cdyn=(K+I)Cf and is slightly modified by an input geometric capacitance Cin connected in parallel thereto. This parallel connection of the detector capacitance Cd and the capacitance Cdyn+Cin results in that the charge q accumulated in the detector in the section αq is transferred to the feedback capacitance Cf. The coefficient α is equal to:
  α  =                    C        wej            +                        (                      K            +            1                    )                ⁢                  C          f                                    C        d            +              C        wej            +                        (                      K            +            1                    )                ⁢                  C          f                    
Because the amplification coefficient K of the amplifier system (without feedback) is very large (typically K=103 . . . 109), then for typical capacitance values, Cf, Cin, Cd (in the range from one to several hundred pF), the coefficient α is close to unity. For this reason, almost 100% of the charge collected by the detector is transferred to the feedback capacitance. The amount of the charge q collected by the detector depends on the size of the detector, its type (the substance used, e.g. gas or solid state-semiconductor), as well as the value of the electrical field, which accumulates the generated charge. To accumulate this charge optimally, relatively high voltage is used for powering the detection system.
The best noise parameters are achieved in systems, wherein the input electronic circuits are based on JFET transistors with the feedback resistor removed. In standard systems of this type, this causes slow charging of the feedback capacitance by a reverse current of the junction. This current cannot be compensated by using drain feedback or optoelectronic feedback. These feedback types only allow controlling of the increase of the reverse current of the junction without the possibility of changing its direction.
The only possibility of compensation is connecting an external power source, in which the direction of current flow (due to accumulation of the charge on capacitance Cf) occurs in the direction opposite to the direction determined by the gate current IG of the JFET transistor. Such situation can occur, when e.g. a semiconductor detector is connected to the input of the integrator. An example of such system is shown in FIG. 2. Still, each such interference into the system increases, usually many times, the input noise level. In case of connecting the semiconductor detector there appears a junction which charges the Cf capacitance with ID current of direction opposite to the direction of the gate current of the transistor.
A similar method of compensating the gate current of transistor T1 is achieved in the system formed of complementary field effect transistors, as shown in FIG. 3, known from the publication of the PCT application WO2012114291. In such systems, the junction which compensates the gate current of the n-channel transistor is a p-channel transistor. In such solution, the additional junction of p-channel transistor virtually does not lower the parameters of the system, and moreover, it tends to improve these parameters, especially in case of systems of high capacitance. This is due to the fact that p-channel transistor participates in a symmetrical way in the process of signal amplification. In this system, the compensation is typically not complete (due to the fact that it is virtually impossible to match junction transistors so that they could have identical gate currents). It would be desirable to provide such a modification of the system that would allow to achieve a full equalization of gate currents.
It would also be desirable to improve the system for measuring the electrical charge so that the system would be particularly useful for accumulating and precisely measuring a slowly varying electrical charge.